Add platforms.slack.extra.rich_blocks (default off). When enabled, the
final agent message is sent as Slack Block Kit blocks — section headers,
dividers, and true nested lists via rich_text — instead of flat mrkdwn.
- New plugins/platforms/slack/block_kit.py: pure markdown->blocks renderer
(headers, dividers, nested ordered/bullet lists, blockquotes, fenced code;
pipe-tables as aligned monospace since Block Kit has no robust table block).
Enforces Slack's 50-block / 3000-char section limits and returns None to
fall back to plain text on empty/oversized/unexpected input. Never raises.
- adapter.send(): render blocks on the single-chunk primary message; a
text= fallback is ALWAYS sent alongside (notifications/accessibility).
- adapter.edit_message(): blocks only on finalize=True, so intermediate
streaming edits stay plain mrkdwn (no per-flush block re-derivation).
- Docs (EN + zh-Hans) + config example. Send-side only: no app reinstall.
Tests: pure-renderer unit suite + adapter integration suite (blocks present
when on, plain text when off, text fallback always set, finalize gating,
multi-chunk fallback). Prove-failed against a stubbed renderer.
